Inhalable insulin is a powdered form of insulin regular, delivered with a nebulizer into the lungs where it is absorbed. Exubera, developed by Inhale Therapeutics (later named Nektar Therapeutics), became the first inhaled insulin product to be marketed in 2006 by Pfizer, but poor sales led Pfizer to withdraw it in 2007. Afrezza, a monomeric inhaled insulin developed by Mannkind, was approved by the FDA in 2014.

Insulin regular is a short-acting insulin. When subcutaneously administered, the onset of action (as evidenced by a decrease in glucose level) occurs 30 minutes post-dose. Maximal effect occurs between 1.5 and 3.5 hours post-dose. The glucose-lowering effect occurs 8 hours post-dose. Compared to other rapid-acting insulin analogs, insulin regular has a slower onset of action and longer duration of action.

Mechanism of action

The primary activity of insulin is the regulation of glucose metabolism. Insulin promotes glucose and amino acid uptake into muscle and adipose tissues, and other tissues except brain and liver. It also has an anabolic role in stimulating glycogen, fatty acid, and protein synthesis. Insulin inhibits gluconeogenesis in the liver.
Insulin binds to the insulin receptor (IR), a heterotetrameric protein consisting of two extracellular alpha units and two transmembrane beta units. The binding of insulin to the alpha subunit of IR stimulates the tyrosine kinase activity intrinsic to the beta subunit of the receptor. The bound receptor is able to autophosphorylate and phosphorylate numerous intracellular substrates such as insulin receptor substrates (IRS) proteins, Cbl, APS, Shc and Gab 1. These activated proteins, in turn, lead to the activation of downstream signaling molecules including PI3 kinase and Akt. Akt regulates the activity of glucose transporter 4 (GLUT4) and protein kinase C (PKC) which play a critical role in metabolism and catabolism.